1. Field of the Invention
This invention relates to devices that generate vibrational energy and to control of that energy. The invention further relates to the use of controlled vibrational energy, especially to the use of controlled vibrational energy to mitigate fouling in heat transfer components (including but not limited to heat exchangers, in particular heat exchangers) used in refineries and petrochemical plants.
2. Discussion of Related Art
Vibration is used in a variety of processes, including manufacturing, particulate flow control, packaging, and testing, for example. Vibration has also been used to prevent particles from settling or accumulating on certain surfaces. One such application is directed to mitigating fouling of equipment due to the build up of material on surfaces that interferes with normal operations of the equipment.
Fouling is generally defined as the accumulation of unwanted materials on the surfaces of processing equipment. In petroleum processing, fouling is the accumulation of unwanted hydrocarbon-based deposits on heat exchanger surfaces. It has been recognized as a nearly universal problem in design and operation of refining and petrochemical processing systems, and affects the operation of equipment in two ways. First, the fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of the heat exchangers—thus increasing temperature in the system. Second, as deposition occurs, the cross-sectional area is reduced, which causes an increase in pressure drop across the apparatus and creates inefficient pressure and flow in the heat exchanger.
Fouling in heat transfer components (including heat exchangers) associated with petroleum type streams can result from a number of mechanisms including chemical reactions, corrosion, deposit of insoluble materials, and deposit of materials made insoluble by the temperature difference between the fluid and heat exchange wall.
One of the more common root causes of rapid fouling, in particular, is the formation of coke that occurs when crude oil asphaltenes are overexposed to heater tube surface temperatures. The liquids on the other side of the heat transfer component are much hotter than the whole crude oils and result in relatively high surface or skin temperatures. The asphaltenes can precipitate from the oil and adhere to these hot surfaces. Prolonged exposure to such surface temperatures, especially in a late-train exchanger, allows for the thermal degradation of the asphaltenes to coke. The coke then acts as an insulator and is responsible for heat transfer efficiency losses in the heat exchanger by preventing the surface from heating the oil passing through the unit. To return the refinery to more profitable levels, the fouled heat exchangers need to be cleaned, which typically requires removal from service, as discussed below.
Heat exchanger in-tube fouling costs petroleum refineries hundreds of millions of dollars each year due to lost efficiencies, throughput, and additional energy consumption. With the increased cost of energy, heat exchanger fouling has a greater impact on process profitability. Petroleum refineries and petrochemical plants also suffer high operating costs due to cleaning required as a result of fouling that occurs during thermal processing of whole crude oils, blends and fractions in heat transfer equipment. While many types of refinery equipment are affected by fouling, cost estimates have shown that the majority of profit losses occur due to the fouling of whole crude oils and blends in pre-heat train exchangers.
Heat exchanger fouling forces refineries to frequently employ costly shutdowns for the cleaning process. Currently, most refineries practice off-line cleaning of heat exchanger tube bundles by bringing the heat exchanger out of service to perform chemical or mechanical cleaning. The cleaning can be based on scheduled time or usage or on actual monitored fouling conditions. Such conditions can be determined by evaluating the loss of heat exchange efficiency. However, off-line cleaning interrupts service. This can be particularly burdensome for small refineries because there will be periods of non-production.
Mitigating or possibly eliminating fouling of heat transfer components can result in huge cost savings in energy reduction alone. Reduction in fouling leads to energy savings, higher capacity, reduction in maintenance, lower cleaning expenses, and an improvement in overall availability of the equipment.
Attempts have been made to use vibrational forces to reduce fouling in heat exchangers. The basis for using vibration is to provide a mechanism by which motion is induced in the liquid in the tubes to disrupt the formation of deposits on the surface of the heat exchanger. It is difficult, however, to efficiently generate and transmit the vibrational energy to the surface of the heat exchanger in a controlled manner.
A vibrational system has been developed by the assignee of this application, ExxonMobil Research and Engineering Company, that utilizes a mechanical force applied to a fixed mounting element that supports heat exchanger tubes for liquid flow to induce a vibration in the tubes that causes shear motion in the liquid flowing adjacent to the tubes to reduce fouling of the tubes. The system is disclosed in co-pending application U.S. Ser. No. 11/436,802 entitled “Mitigation of In-Tube Fouling in Heat Exchangers Using Controlled Mechanical Vibration” filed May 19, 2006. The contents of that application are incorporated herein by reference.
Other methods of generating vibration include using electromagnetic devices or piezo-electric shakers, which would allow a greater degree of control of frequency and amplitude. However, these types of devices pose a number of problems in refinery settings. They are high in cost, low in reliability in harsh environments, and can raise safety concerns due to the high electric power needed to drive these devices.
An alternative would be a pneumatic vibrator, which is lower in cost, more reliable and safe. Pneumatic vibrators per se are well known. Typically, they operate by generating vibration due to centrifugal force of either rotary ball motion or rotation of an unbalanced turbine when driven by compressed air or gas. The frequency and amplitude of vibration usually increase with the pressure and flow. However, the problem with pneumatic vibrators is that it is difficult to control the frequency and amplitude of such devices. It is particularly difficult to control the frequency and amplitude independently of each other.
There is a need to develop additional methods for reducing in-tube fouling, particularly methods that can enhance control of the energy used to reduce fouling. There is also a need to design pneumatic vibration generation systems that can be more closely controlled, particularly devices in which the frequency and amplitude can be independently controlled.